Ceramic composites consist of ceramic fibers (usually in the range 10 to 20 μm diameter and often woven into fabrics) and a matrix which is also ceramic. When made correctly they are strong, tough materials that can be used in a wide range of high temperature structural components, including engines (power generation turbines, aircraft engines, rocket engines, hypersonic engines), hot airframe structures, and thermal protection systems.
The choice of materials for the fibers and matrix is usually driven by the high temperature chemical environment of the application and lifetime requirements. For short lifetimes and very high temperatures, carbon or SiC are generally chosen. For long lifetimes in oxidizing environments, oxide materials are generally chosen since they are thermodynamically stable in the environment.
An important feature required in order to make a tough ceramic composite is some mechanism for decoupling or segregating damage in the matrix and fibers. Without this ability to control and mitigate damage within the matrix and fibers, cracks grow directly from matrix to fibers and the composite is brittle. In oxide composites, two methods have been used to decouple fiber and matrix fracture: a) use of a matrix material with sufficiently low strength that damage is unable to propagate from the matrix to the fibers, and b) use of a material in the matrix (or as a coating on the fibers), which does not bond strongly to the fibers.
With respect to the first method—the use of a matrix material with sufficiently low strength that damage is unable to propagate from the matrix to the fibers—a sufficiently low strength is usually achieved by using a matrix that contains a significant level of porosity, either distributed throughout the matrix, or concentrated in a porous layer around individual fibers. This type of composite is analogous to a starched fabric. For this mechanism to be successful, the volume fraction of porosity in the matrix must be larger than a specified critical value.
With respect to the second method—the use of a material in the matrix (or as a coating on the fibers), which does not bond strongly to the fibers—the matrix itself can have high strength, while the weak interface between the fibers and matrix prevents propagation of damage from the matrix to the fibers, as needed for high toughness. The strength of a matrix can be made high by minimizing the volume fraction of porosity. A well-defined criterion exists for the upper limit on the bond strength between the fiber and matrix (or coating) for this method to succeed in providing toughness and damage tolerance in the composite.
The second method listed is generally preferred. A high matrix strength is beneficial in composites formed by laminating layers of reinforcing fibers (which may be in the form of woven fabrics, layers of aligned fibers, or mats of fibers) since the interlaminar strength of the composite is provided solely by the matrix. Composites with porous weak matrices have high strength and toughness when loaded in the direction of the fibers but low interlaminar strengths, whereas composites with dense strong matrices can have high interlaminar strengths. A low volume fraction of porosity in the matrix is also beneficial in preventing the ingress of reactive or damaging liquids and gases into the composite during use.
It is generally difficult to produce ceramic composites with a fully dense matrix. The reason for this difficulty is that most dense ceramics are formed by sintering of powders, a process that involves shrinkage; and when the ceramic is surrounded by rigid fibers, as in the matrix of a composite, the shrinkage needed for densification is prevented by the constraint of the fibers. However, a method to achieve a dense matrix would give considerable benefit, as described above.
Dense matrices can be formed in ceramic composites containing non-oxide reinforcing fibers by infiltrating the matrix material in a molten form. An example is the infiltration of molten silicon into fiber preforms of carbon or silicon carbide. However, this approach is not useful for composites containing oxide fibers, because all available oxide fibers that are suitable for production of composites are degraded by creep and grain growth at the high temperatures needed to melt the matrix materials that are of interest.
Attempts in the literature to produce oxide composites with dense matrices have started with porous-matrix composites, which can be fabricated by several methods, all giving volume fractions of porosity above approximately 50% in the matrix. Two methods have been used to reduce this porosity and thus strengthen the matrix. The first is by application of pressure to the composite at high temperature (this being a common method for densifying monolithic ceramic materials). However, with all available oxide fibers that are suitable for production of composites, degradation of the fibers by creep and grain growth occurs, and therefore, limits the usefulness of this approach.
The second method involves infiltrating the porous-matrix composite at low temperature with a liquid precursor that forms the desired matrix material after being heated at a higher temperature. An example is the infiltration of an aqueous solution of aluminum oxychloride into a porous matrix of Al2O3, followed by heat treatment in air to temperatures above 1000° C. (See Levi C G, Zok F W, Yang J Y, Mattoni M, Lofvander J P A: Microstructural design of stable porous matrices for all-oxide ceramic composites. Zeitschrift fur metallkunde 1999, 90: 1037-1047). During the heat treatment, the aluminum oxychloride decomposes to form Al2O3. Since the decomposition is accompanied by volume shrinkage, multiple cycles of infiltration and heat treatment are needed to eliminate porosity completely. Although this method can densify the matrix successfully, all previous attempts in the literature have produced matrices strongly bonded to the fibers with resulting embrittlement and reduction in strength of the composite.
The only materials known to serve the purpose of bonding weakly to oxide fibers, while possessing other properties required for use in high temperature composites (high melting point, chemical compatibility with other oxides), are rare-earth phosphates (monazite and xenotime). (See Morgan, P. E. D. and Marshall, D. B., “Ceramic Composites having a Weak Bond Material Selected from Monazites and Xenotimes”, U.S. Pat. No. 5,514,474 (1996). [Europe, France and UK: #0677497, Germany: #P69506304, Japan: #2901895]; Morgan, P. E. D. and Marshall, D. B., “Fibrous Composites including Monazites and Xenotimes” continuation in part, U.S. Pat. No. 5,665,463 (1997); Davis, J. B., Marshall, D. B., Oka, K. S., and Morgan, P. E. D., “Monazite-based Blanket Coatings for Thermal Protection Systems” U.S. Pat. No. 6,716,407 (2004), which are all incorporated herein by reference in their entirety.) Monazites are a family of phosphates having the general formula MPO4 (MP), where M is selected from the larger trivalent rare earth elements of the lanthanide series, such as lanthanum (La), cerium (Ce), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb) or combinations thereof and coupled substituted divalents and tetravalents such as calcium or strontium with thorium. Xenotimes are phosphates similar to monazite, where M is selected from Sc, Y, and the smaller trivalent rare-earth elements of the lanthanide series (e.g., including dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu) or combinations thereof.
Therefore, it would be ideal to produce ceramic-matrix composites in which the fibers are surrounded by monazite or zenotime material and in which the matrix is produced with a method that meets at least one of the following goals: a) produces a dense matrix, b) produces a higher interlaminar strength as compared with conventional porous matrix composites, c) can be easily tailored to provide varying degrees of strength and matrix properties, d) can be formed by strengthening conventional oxide composite materials and e) maintains significant toughness for in-plane loading.